A hand of an industrial robot requires at least 2 fingers and also requires that the fingers thereof can selectively take three independent positions, namely: (a) an open position in which the fingers are open to prepare for receipt of an object which is handled by the industrial robot; (b) a closed position in which the fingers are closed and firmly grip the object which is handled by the industrial robot; and (c) an intermediate position or a neutral position in which the fingers are brought to and maintained at a neutral position which can be arbitrarily selected between the open position and the closed position. The reason a robot hand is required to take an intermediate position is to enable an industrial robot to be taught to cause the arm thereof to move to a position in which the fingers thereof contact each other at a position approximately corresponding to the center of an object which is handled by the industrial robot, when a robot hand is preliminarily brought to a grip position (a position in which an object is placed to be gripped by a robot hand) prior to commencement of a probably consecutive operation. Incidentally, it is important to note that the fingers stay apart from each other at an intermediate position, albeit the distance between the fingers is adjustable. In other words, there are possibilities in which a sufficient gripping force can not necessarily be expected from the fingers, particularly for an object having a small size, at an intermediate position.
Fingers of a robot hand are preferably driven by a mechanism which comprises a double acting cylinder, of which an example having two fingers is shown in FIG. 1. Referring to FIG. 1, fingers 1 and 1' grip an object 2 which is handled and/or carried by an industrial robot. The fingers 1 and 1' are driven by a link 3 which tilts around a supporting point 3'. The link 3 is driven by a piston 4' which moves up and down in FIG. 1 along the internal surface of a double acting cylinder 4, which is supplied working liquid pressure through a set of two liquid inlets 5 and 5', either of which is selectively connected with a working liquid pressure source (not shown).
A working liquid is preferably supplied to the double acting cylinder 4 through one or more electromagnetic valves, more specifically through two, independent, 2-position, 5-port-type, single solenoid, electromagnetic valves. A 2-position, 5-port-type, single solenoid, electromagnetic valve, of which one example is shown in FIGS. 2(a) and 2(b), is defined as an electromagnetic valve which comprises: (a) an inlet 6 through which a working liquid is supplied to the valve; (b) two outlets 7 and 7' either of which is, in response to the ON or OFF position of the solenoid of the valve, connected with the inlet 6 through a path prepared in the valve; and (c) two purging holes 8 and 8' either of which is connected with one of the outlets 7 and 7' which is not connected to the inlet 6 through a path prepared in the valve, thereby the connection of the liquid paths is switched in response to the ON or OFF position of the solenoid of the valve. It is to be assumed in this specification that a 2-position, 5-port-type, single solenoid, electromagnetic valve has two independent functional positions, either of which is taken in response to the ON or OFF position of a solenoid as respectively shown in FIGS. 2(a) and 2(b), including a first position at which the inlet 6 and the outlet 7 are connected through a path shown by an arrow A and the outlet 7' and the purging hole 8' are connected through a path shown by an arrow B in response to the ON position of the solenoid, and a second position at which the inlet 6 and the outlet 7' are connected through a path shown by an arrow C and the outlet 7 and the purging hole 8 are connected through a path shown by an arrow D in response to the OFF position of the solenoid.
In the prior art, two, independent, 2-position, 5-port-type, single solenoid, electromagnetic valves are connected with a double acting cylinder, as shown in FIG. 3, to constitute a driving means of a robot hand which enables a robot hand to take an intermediate position. Referring to FIG. 3, an inlet 15 of a double acting cylinder 14 is connected with an outlet 17' of a first electromagnetic valve 10, and an inlet 15' of the double acting cylinder 14 is connected with an outlet 27' of a second electromagnetic valve 20. These outlets 17' and 27' of the first and second electromagnetic valves 10 and 20 are connected respectively with an inlet 16 of the first electromagnetic valve 10 and with an inlet 26 of the second electromagnetic valve 20 in response to the OFF position of the solenoid thereof (See FIG. 2(b)). In response to the OFF position of the solenoids of the first and second electromagnetic valves 10 and 20, the inlet 15 of the double acting cylinder 14 is connected with a working liquid pressure source 9 through the outlet 17' and the inlet 16 of the first electromagnetic valve 10, and the inlet 15' of the double acting cylinder 14 is also connected with the working liquid pressure source 9 through the outlet 27' and the inlet 26 of the second electromagnetic valve 20, resultantly supplying working liquid pressure to both chambers of the double acting cylinder 14. In contrast, in response to the ON position of the solenoids of the first and second electromagnetic valves 10 and 20, the outlet 17' is connected with a purging hole 18' and the outlet 27' is connected with a purging hole 28', resultantly reducing the internal pressure of both chambers of the double acting cylinder 14 to zero. The other outlets 17 and 27 are plugged to inhibit the working liquid pressure source 9 from directly being connected with the atmosphere.
The fingers of a robot hand are opened or closed by making the solenoid of one electromagnetic valve ON or OFF and the solenoid of the otherr electromagnetic valve OFF or ON, resultantly driving a piston 14' of the double acting cylinder 14 upward or downward in FIG. 3. In contrast, the fingers are brought to and maintained at an intermediate position by making the solenoids of the both electromagnetic valves 10 and 20 OFF to supply working liquid pressure to both chambers of the double acting cylinder 14, resultantly driving the piston 14' toward the middle part of the double acting cylinder 14 and holding the piston 14' at a location arbitrarily selected between the open and closed positions.
The aforementioned robot hand available in the prior art includes the drawback wherein a termination of electric power supply brings the robot hand to an intermediate position in which the gripping force of the fingers of the robot hand is not necessarily reliable, particularly for a small object, resultantly causing the possibility that the robot hand will fail to keep holding an object, in the event of an electric power failure. In other words, the robot hand available in the prior art does not necessarily satisfy the requirement that the fingers of a robot hand preferably shift to a safer position in response to a termination of electric power supply.